This invention is directed to a control technique for vibratory conveyors and, more particularly, to a method and system for controlling a vibratory conveyor using pulse width modulated drive signals, vibratory amplitudes and resonant frequency control.
Vibratory conveyors are used industrially for moving products of different shapes and weights from one location to another. For example, in the packaging of fragile food products such as potato chips or cookies, the food product is received from a central location, such as a cooking oven, and conveyed to a plurality of work stations having packaging machines. The processing of produce (e.g., fruits and vegetables) similarly requires the handling of fragile food products. Vibratory conveyors are especially useful in such applications because such fragile food products may not readily be transported in other ways without damaging the products.
Despite the usefulness of vibratory conveyors in these types of applications, some existing systems still lack effective methods for controlling vibration amplitude and frequency. In open loop control systems, for example, vibration intensity is often controlled simply by using a variable transformer or some other means for manually regulating a voltage source. However, such systems tend to be sensitive to line voltage variations, load changes, spring wear and other dynamic conditions that result in inconsistent control of the vibration envelope.
In closed loop configurations, vibratory conveyors tend to be driven at their resonant frequency and employ sensors for acquiring vibration amplitude feedback. However, closed loop configurations of vibratory conveyors tend to require controllers that are expensive and complex to implement. In particular, closed loop systems tend to use expensive and complicated linear drive circuits and control techniques that consume excessive power. Frequently, such linear drive circuits are based on MOSFET (Metal Oxide Semiconductor Field Effect Transistor) circuit technology, which can result in costly implementations of closed loop vibratory conveyor control systems.
Vibration frequency in certain existing closed loop control systems is controlled by applying an energizing pulse to a drive coil only during one-quarter of the path of travel of a conveyor pan on a vibratory conveyor. These systems provide very limited, and often no ability to regulate the vibration frequency or vibration amplitude of the path traveled by a conveyor pan on a vibratory conveyor. The rather limited ability to control vibration amplitude or vibration frequency is directly related to the limited ability to provide an enabling control signal to a current switch that can be used to deliver an electrical current to a drive coil coupled to a vibratory conveyor. Without the ability to effective control the current switch, such vibratory conveyor systems cannot regulate the period or frequency of motion of a conveyor pan on a vibratory conveyor.
Controlling vibration amplitude is yet another challenge in existing vibratory conveyor control systems. In such systems, a peak vibration amplitude signal measured by a motion sensor must often be compared with a user-selected amplitude value. Vibration amplitude often can only be adjusted based on such comparisons made at the start of an energizing pulse to a drive coil and for the one-quarter of a period through which the coil may be energized. Such systems provide limited opportunities to control vibration amplitude over the entire period, or even one-half of the period, over which a current switch is enabled.
Therefore, a need exists for a closed loop control system for a vibratory conveyor that provides greater control over the generation and switching of electrical current for regulating the vibration amplitude and vibration frequency of a conveyor pan mounted on a vibratory conveyor. In such a system, the sensing, signal processing, translating and controlling capabilities must be implemented using a minimum amount of electrical circuitry to reduce total manufacturing costs and total power consumption, while providing significantly greater control over vibration amplitude, vibration frequency and reduced physical stress on a vibratory conveyor.
The present invention contemplates, in one embodiment, a system for controlling a vibratory conveyor on which a conveyor pan is mounted, the system including a drive coil for driving the vibratory conveyor with a vibrating motion; a sensing subsystem for detecting the vibrating motion of the conveyor pan; a controller coupled to the sensing subsystem for generating at least one control signal to be applied to the drive coil, the controller generating at least one control signal by producing a digitized signal from the detected vibrating motion and computing three intermediate timing positions in each period of the digitized signal; and a switching subsystem for applying at least one control signal generated by the controller to the drive coil. At least one control signal is applied between the first and third intermediate timing positions in each period of the digitized signal.
The present invention also contemplates a method used in the system for controlling a vibratory conveyor having a conveyor pan mounted thereon and a drive coil. The method involves sensing the vibration motion of the conveyor pan on the vibratory conveyor and producing a periodic vibration signal based on the sensed vibration motion. Additional steps in the method involve processing the periodic vibration signal, generating at least one control signal based on the processed periodic vibration signal and driving the vibratory conveyor with a drive current based on at least one control signal. The drive current is applied to the vibratory conveyor from a first intermediate point to a second intermediate point, the first intermediate point representing a point farthest from the drive coil and the second intermediate point representing a point closest to the drive coil.
The present invention provides a vibratory conveyor control system having significantly reduced noise since an insulated gate bipolar transistor (IGBT) is used in place of a pulsed silicone controlled rectifier (SCR). A pulsed SCR provides a short, sharp pulse of electrical current that can physically stress the vibratory conveyor system. In its place, an IGBT is substituted that provides electrical current over longer periods of time without the necessity to deliver current in sharp pulses. The use of IGBT""s also significantly reduces the manufacturing cost of the system. A seventy-five percent reduction in manufacturing cost can be achieved by implementing the system with IGBT""s instead of SCR""s.
The present invention also provides significantly less physical stress to the vibratory conveyor control system. In particular, the mechanical stress on the system is reduced since the drive coil applies electrical current to the vibratory conveyor when the conveyor pan 102 mounted on the vibratory conveyor is farthest from the drive coil. An increasing level of electrical current is applied between the point farthest from the drive coil to a crossover point midway between the farthest point and the drive coil. After the conveyor pan crosses the crossover point, the drive coil applies progressively less electrical current until the point at which the conveyor pan reaches a point of closest approach to the drive coil. By regulating the delivery of electrical current to the vibratory conveyor in this manner, the physical stress on the vibratory conveyor is reduced while pulling the conveyor pan from a point farthest from the drive coil to a point closest to the drive coil.
The system and method comprising the present invention provide greater control over vibration frequency and vibration amplitude of a vibratory conveyor. Greater control of these factors is made possible since the system regulates the amount of electrical energy instead of the time intervals in which electrical current can be switched to the drive coil. The invention also provides significant manufacturing cost savings and greatly reduces the physical stress on a vibratory conveyor.